Overload switch

Abstract

In a transformer primary circuit breaker having a primary switch for opening and closing the transformer primary circuit, an operating handle mounted exteriorly of the transformer for opening and closing the primary switch, a trip mechanism operatively connected to open the primary switch under fault current conditions, the trip mechanism including a conductive metallic sensor element connected in the primary circuit and having a predetermined Curie temperature and a magnet mounted for movement into engagement with the sensor element and biased to open the primary switch when the temperature of the sensor element approaches the Curie temperature, the improvement comprising a shunt circuit connected to the primary circuit in parallel with the sensor element, a normally closed switch in the shunt circuit, a resistance element for limiting the current passing through the shunt circuit, and a tab mounted on the operating handle for opening the shunt circuit switch, a spring mounted in the operating handle for limiting the motion of the operating handle to thereby close the shunt switch, whereby a portion of the primary current will flow through the shunt circuit when the switch is closed. A bimetal switch can be included in the shunt circuit to open the shunt circuit at predetermined oil temperatures or excessive currents.

I claim: 
     
       1. In a primary circuit breaker for a transformer, said circuit breaker including a frame, a primary switch mounted on said frame for opening and closing the primary circuit, an operating handle mounted exteriorly of the transformer for opening and closing the primary switch, a trip mechanism operatively connected to open the primary switch under fault current conditions, the trip mechanism including a conductive metallic sensor element connected in the primary circuit and having a predetermined Curie temperature and a magnet mounted for movement into engagement with the sensor element and biased to open the primary switch when the temperature of the sensor element approaches the Curie temperature, the improvement comprising a shunt circuit connected to the primary circuit in parallel with said sensor element, a normally closed switch in said shunt circuit and means mounted in said operating handle for selectively opening said shunt circuit switch whereby a portion of said primary current will flow through said shunt circuit when said switch is closed. 
     
     
       2. The improvement according to claim 1 wherein said shunt circuit includes a resistance element for limiting the current passing through the shunt circuit to a percentage of the current in the primary circuit. 
     
     
       3. The improvement according to claims 1 or 2 wherein said opening means includes a spring for limiting the rotary motion of said operating handle and a tab mounted on said operating handle in a position to open said shunt circuit switch depending on the position of said operating handle. 
     
     
       4. The improvement according to the claims 1 or 2 wherein said shunt circuit includes a bimetal switch positioned to respond to the transformer oil temperature. 
     
     
       5. The improvement according to claims 1 or 2 including temperature responsive means in said shunt circuit for opening said shunt circuit at excessive temperatures. 
     
     
       6. In an underoil primary circuit breaker for a transformer, the circuit breaker including a magnetic, heat sensitive trip assembly for opening the circuit breaker, the assembly including a sensor element having a predetermined Curie temperature, the improvement comprising a shunt circuit assembly for bypassing a portion of the primary current passing through the sensor element for the trip assembly for emergency overload service, said shunt circuit assembly including a resistance element for controlling the proportion of current passing through the shunt circuit assembly and a shunt switch for opening and closing the shunt circuit assembly. 
     
     
       7. The shunt circuit assembly according to claim 6 including means operable externally of the transformer for selectively opening or closing said switch. 
     
     
       8. The shunt circuit assembly according to claims 6 or 7 including a bimetal switch operatively positioned to respond to the temperature of the oil in the transformer. 
     
     
       9. The shunt circuit assembly according to claims 6 or 7 including a bimetal switch positioned to respond to the temperature of the oil in the transformer and to current flow through the bimetal switch. 
     
     
       10. In a primary circuit breaker for a transformer of the type having a crank for opening and closing the primary circuit breaker and an operating handle mounted on said crank external to the transformer, a trip mechanism operatively connected to open the circuit breaker under fault current conditions, the trip mechanism including a sensor element connected in the primary circuit and a magnet mounted for movement into engagement with said element and biased to actuate the trip mechanism when the temperature of sensor element approaches the Curie temperature, the improvement comprising a shunt circuit connected across the sensor element, a switch in said shunt circuit, means in said shunt circuit for controlling the proportion of primary current bypassed through said shunt circuit, and means mounted in the operating handle for selectively opening and closing said shunt circuit switch. 
     
     
       11. The improvement according to claim 10 wherein said controlling means comprises a resistance element. 
     
     
       12. The improvement according to claim 10 or 11 wherein said opening and closing means comprises a tab mounted on the crank in a position to open and close said shunt circuit switch depending on the rotary position of the crank, said means in the operating handle limiting the rotary motion of the crank. 
     
     
       13. The shunt circuit assembly according to claims 10 or 11 including a bimetal switch connected to the shunt circuit and operatively positioned to respond to the temperature of the oil in the transformer to open the shunt circuit. 
     
     
       14. The shunt circuit assembly according to claims 10 or 11 including a bimetal switch connected to open said shunt circuit in response to a predetermined oil temperature or an increase in current.